Pm me?
I'm having that issue a fair bit.
It's as some people have been mentioning - either Minecraft needs more RAM, or it needs less RAM. (I suggest looking up a guide on how to give more or less RAM to Minecraft if you want to do this, as I'm not going to explain it myself.

Needing less should only be an issue if you've added some in the past.)
The problem is that 32 bit operating systems, to my knowledge, are not able to allocate anymore than 1.5 GB of RAM to Minecraft. They also handle RAM much differently. When on a 32 bit operating system, Minecraft will even warn you that far render distance is not recommended upon switching to the setting, (not sure if it happens when you have optifine.) which is rightly so as Minecraft will frequently crash with "out of memory" when doing so, which it will not do on a 64 bit with the same memory allocated. Sadly...I could not, no matter how much I looked, find a solution to the errors anywhere, other than "allocate more memory/RAM," which is something I was of course unable to do until I got my new computer. Somehow, though, my problem magically fixed itself...I swear, I did nothing. (Still got lots of lag and invisible chunks in SG2 by refill, though.) >.>
Right now, I believe I have...4? GB of memory allocated to Minecraft, which is admittedly probably more than I need. I'm experimenting with it a bit until I find a good place to maximize it's performance. (It does feel a bit slower than when I had 3, so I'll probably go back to that.)
For the most part, any guide to allocating more memory will include things like how to figure out what operating system you use and things of the sort, so I'll leave it to that. This may be the solution you need. (possibly for BritishShoop as well, thus why I'm posting it instead of PM'ing it.)
As a side note: a small bit of false and/or ignorant assumptions have possibly been made in this post. I'm no computer nerd, this is just what I learned from trying to fix my problems in the past. For the most part, though, any important information (like how to actually do it.) I left to the people who know what they're talking about who made the guides on the internet.
